Problem 188 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following the most basic compound is \([2005]\) (a) benzylamine (b) aniline (c) acetanilide (d) \(\mathrm{p}\)-nitroaniline

Step-by-Step Solution

Answer

Benzylamine is the most basic compound among the given options.

Step 1: Understand Basicity

Basicity in organic compounds usually depends on the availability of a lone pair of electrons on a nitrogen atom. The more available the lone pair is for donation, the more basic the compound will be.

Step 2: Analyze Compounds' Functional Groups

We have four compounds: benzylamine, aniline, acetanilide, and p-nitroaniline. In benzylamine, the lone pair on the nitrogen is relatively free; in aniline, the lone pair is partially delocalized into the benzene ring, reducing its availability; in acetanilide, the nitrogen is part of an amide group, and here the lone pair is involved in resonance with the carbonyl group; in p-nitroaniline, the nitro group (-NO2) is an electron-withdrawing group, making the lone pair less available.

Step 3: Compare the Compounds' Basicity

Among the given compounds, benzylamine is the most basic because its nitrogen's lone pair is not involved in resonance with the benzene ring or any other functional group, making it highly available to accept protons. Aniline is less basic due to resonance stabilization, acetanilide has even less basic character due to its amide group, and p-nitroaniline is the least basic because of the electron-withdrawing effect of the nitro group.

Step 4: Conclusion

Based on the analysis, benzylamine has the highest availability of the nitrogen lone pair for donation and thus is the most basic of the options.

Key Concepts

Benzylamine

Benzylamine, a primary amine, is an organic compound with an amine group attached to a benzyl group. The structural importance of benzylamine lies in the presence of a nitrogen atom that bears a lone pair of electrons, which is not shared or involved in resonance with any other group. This availability makes the lone pair highly accessible for donation, rendering benzylamine a very basic compound.

• The presence of the benzyl group does not delocalize the electron pair, keeping it free for proton acceptance.
• In the context of basicity, benzylamine’s nitrogen atom shows a high tendency to bind to protons effectively.

Due to these characteristics, benzylamine is often cited as more basic than compounds where lone pair electron resonance is possible.

Aniline

Aniline, a simple aromatic amine, features a nitrogen atom attached directly to a benzene ring. This structural orientation has significant implications for its basicity. In aniline, the lone pair of electrons on the nitrogen can participate in resonance, which is the distribution of the electron density throughout the aromatic ring.

• This resonance tends to stabilize the molecule but also results in reduced availability of the lone pair for protonation, decreasing aniline's basicity compared to non-aromatic amines like benzylamine.
• The electron density is shared with the ring, weakening the nitrogen's electron-donating ability.

Hence, despite being basic, aniline is relatively less basic than benzylamine due to this delocalization of electrons.

Amide Group

The amide group is a functional group characterized by a nitrogen atom bonded to a carbonyl group (C=O). This structure is significant because the lone pair on the nitrogen engages in resonance with the carbonyl, contributing to its decreased basicity.

• This resonance between the lone pair and the carbonyl group reduces the amide's ability to donate electrons readily.
• Consequently, amides, such as acetanilide, are less basic compared to amines where the nitrogen is not part of an amide linkage.

By understanding the electronic structure of amides, one can appreciate their relatively low basicity in organic chemistry contexts.

Electron-Withdrawing Group

Electron-withdrawing groups (EWGs) are critical in influencing the basicity of compounds. An example is the nitro group (-NO2), which pulls electron density away from the rest of a molecule. Their presence can decrease the electron-donating ability of a lone pair on nearby nitrogen atoms by drawing away its electron density.

• In compounds like p-nitroaniline, the nitro group greatly decreases the basicity of the amine by reducing the availability of electrons for protonation.
• Through inductive and resonance effects, EWGs destabilize the positive charge of a protonated nitrogen, hence diminishing basic character.

Understanding the impact of electron-withdrawing groups is key to predicting and explaining the comparative basicity of various organic compounds, especially when these groups are adjacent to or near the functional groups like amines.